(1) Field of the Invention
The present invention relates to a laser discharge tube used in a laser oscillator and a method of manufacturing electrodes of a laser discharge tube, and more specifically, to a laser discharge tube by which the damage of electrodes disposed on the outside periphery of the tube wall of the discharge tube is reduced and a method of manufacturing electrodes of a laser discharge tube.
(2) Description of the Related Art
A discharge tube used in a laser oscillator induces discharge in response to a high voltage applied thereto and outputs a laser beam produced by a laser gas excited by the discharge to the outside. Incidentally, the high voltage is applied to electrodes adhered to the outside periphery of the tube wall of the discharge tube and the discharge takes place between the electrodes confronting each other across the discharge tube. Therefore, discharge normally takes place within a region determined by the width of the electrodes. The discharge tube is composed of silica glass and the electrodes are composed of silver or the like which has good conductivity. The electrodes are formed by adhering silver to the outside wall of the discharge tube by metalizing or the like.
Nevertheless, since a very high high-frequency voltage (e.g., 4000 V) is applied to the electrodes, the electrodes generates a considerable amount of power consumption. That is, since laser output from a laser oscillator is repeatedly turned on and off, the electrodes are repeatedly heated and cooled, by which strain stress is produced between the silica glass constituting the discharge tube and the metalized silver constituting the electrodes. Thus, the electrodes are deteriorated and peeled in a certain life time. A main factor for producing such strain stress is a difference between a coefficient of the silica glass tube and that of the metalized silver. More specifically, since the coefficients of thermal expansion are greatly different, the electrodes are lifted up by a heat cycle resulting from the turning on and off of a laser output so that the electrodes are deteriorated and peeled.
Further, when a very high high-frequency voltage is applied to the electrodes, there is a possibility that dielectric breakdown is caused in regions other than the region between the electrodes and Corona discharge takes place. The Corona discharge is liable to take place at the portion of the discharge tube having a high temperature such as, for example, at the downstream of a laser gas flowing in the discharge tube.
When a high voltage is applied to the electrodes, since the high voltage flows between the electrodes and the outside wall of the discharge tube, breakdown is caused at the edge portion of the electrodes and Corona discharge takes place. The Corona discharge travels once along the surface of the outside wall of the discharge tube and then gets into the discharge tube at a position apart from the electrodes by several millimeters. At the time, the silver is also melted and flown out and the flown-out silver grows to a tree-branch-shape along the outside wall of the discharge tube from the electrodes and covers the outside wall of the discharge tube in the vicinity of the electrodes with a width of several centimeters. When such a phenomenon occurs in which the silver serving as the electrode material migrates (so called an electro-migration), a dielectric strength is further lowered and Corona discharge is more liable to take place, and as a result, the flow-out of the electrode material and deterioration of the electrodes, and the like become more significant.
That is, when the coefficients of thermal expansion has different values, the electrodes are lifted up by the heat cycle and deteriorated and peeled. Corona discharge is more liable to take place in the deteriorated portions and the peeled portions of the electrodes. Therefore, the deterioration of electrodes which start once is accelerated by Corona discharge.
As described above, thermal expansion and Corona discharge caused by power consumption is made more significant when a temperature increases, by which the deterioration and peeling of electrodes are accelerated. Although the deterioration and the like of the electrodes can be prevented by lowering a voltage applied thereto, power to be supplied is lowered accordingly. Thus, a laser beam machining requiring a large amount of power cannot be executed. Further, although there is a method of lowering the temperature of the electrodes by mounting heat radiation plates on the electrodes, thermal expansion arising on the surface where the electrodes are adhered to the discharge tube cannot be avoided by this method. Thus, Corona discharge caused at the downstream of a laser gas where the temperature of the discharge tube is raised to high cannot be also prevented.